Disc drives read and write information along concentric tracks formed on discs. To locate a particular track on a disc, disc drives typically use embedded servo reference marks on the disc. These embedded fields are utilized by a servo subsystem to position a head over a particular track. The servo reference marks are written onto the disc when the disc drive is manufactured and are thereafter simply read by the disc drive to determine position.
Ideally, a head following the center of a track moves along a perfectly circular path around the disc. However, two types of errors prevent heads from following this ideal path. The first type of error is a written-in error that arises during the creation of the servo reference marks. Written-in errors occur because the write head used to produce the servo reference marks does not always follow a perfectly circular path due to unpredictable pressure effects on the write head from the aerodynamics of its flight over the disc, and from vibrations in the gimbal used to support the head. Because of these written-in errors, a head that perfectly tracks the path followed by the servo write head will not follow a circular path.
The second type of error that prevents circular paths is known as a track following error. Track following errors arise as a head attempts to follow the path defined by the servo reference marks. The track following errors can be caused by the same aerodynamic and vibrational effects that create written-in errors. In addition, track following errors can arise because the servo system is unable to respond fast enough to high frequency changes in the path defined by the servo reference marks.
Written-in errors are often referred to as repeatable run-out errors because they cause the same errors each time the head passes along a track. As track densities increase, these repeatable run-out errors begin to limit the track pitch. Specifically, variations between the ideal track path and the actual track path created by the imperfectly-placed servo reference marks can result in a track interfering with (or “squeezing”) an adjacent track. This is especially acute when a first written-in error causes a head to be outside of an inner track's ideal circular path and a second written-in error causes the head to be inside of an outer track's ideal circular path. To avoid limitations on the track density, systems that evaluate and/or correct for repeatable run-out errors are employed.
A technique for repeatable run-out error compensation involves storing time-domain compensation values in the form of a compensation table on discs in the disc drive. These compensation values are injected into the servo loop to compensate for repeatable run-out errors. For example, “Zero Acceleration Path” (ZAP) compensation is so named because it adjusts each track's shape to be very circular, greatly reducing the need for accelerating a transducer head in track following.
Unfortunately, ZAP and similar schemes are very expensive to implement on each data handling system. To evaluate whether or which such schemes are necessary for a given zone, surface, disc drive, or product line, an evaluation of inter-track “squeeze” can be used. One problem with squeeze evaluation systems is that the measurements they generate are not readily repeatable. One way to address this problem is by ignoring frequency components at or below twice the spindle frequency, when computing squeeze indicators. At higher track densities, this is not acceptable. What is needed is a scheme for generating indicators of squeeze that takes low frequency components into account.